diff --git a/src/model/trip.rs b/src/model/trip.rs
index cee6606..a8d4e16 100644
--- a/src/model/trip.rs
+++ b/src/model/trip.rs
@@ -30,12 +30,12 @@ pub struct Trip {
 pub struct TripWithUserAndType {
     #[serde(flatten)]
     pub trip: Trip,
-    rower: Vec<Registration>,
+    pub rower: Vec<Registration>,
     trip_type: Option<TripType>,
 }
 
 impl TripWithUserAndType {
-    async fn from(db: &SqlitePool, trip: Trip) -> Self {
+    pub async fn from(db: &SqlitePool, trip: Trip) -> Self {
         let mut trip_type = None;
         if let Some(trip_type_id) = trip.trip_type_id {
             trip_type = TripType::find_by_id(db, trip_type_id).await;
@@ -230,6 +230,9 @@ WHERE day=?
             }
         }
 
+        let trip_details = TripDetails::find_by_id(db, trip_details_id).await.unwrap();
+        trip_details.check_free_spaces(db).await;
+
         Ok(())
     }
 
diff --git a/src/model/tripdetails.rs b/src/model/tripdetails.rs
index 6eafa88..ec59386 100644
--- a/src/model/tripdetails.rs
+++ b/src/model/tripdetails.rs
@@ -4,6 +4,11 @@ use rocket::FromForm;
 use serde::{Deserialize, Serialize};
 use sqlx::{FromRow, SqlitePool};
 
+use super::{
+    notification::Notification,
+    trip::{Trip, TripWithUserAndType},
+};
+
 #[derive(FromRow, Debug, Serialize, Deserialize)]
 pub struct TripDetails {
     pub id: i64,
@@ -64,6 +69,57 @@ WHERE day = ? AND planned_starting_time = ?
         .await.unwrap()
     }
 
+    /// This function is called when a person registers to a trip or when the cox changes the
+    /// amount of free places.
+    pub async fn check_free_spaces(&self, db: &SqlitePool) {
+        if !self.is_full(db).await {
+            // We still have space for new people, no need to do anything
+            return;
+        }
+
+        let other_trips_same_time = Self::find_by_startingdatetime(
+            db,
+            self.day.clone(),
+            self.planned_starting_time.clone(),
+        )
+        .await;
+
+        for trip in &other_trips_same_time {
+            if !trip.is_full(db).await {
+                // There are trips on the same time, with open places
+                return;
+            }
+        }
+
+        // We just got fully booked and there are no other trips with remaining rower places. Send
+        // notification to all coxes which are registered as non-cox.
+        for trip_details in other_trips_same_time {
+            let Some(trip) = Trip::find_by_trip_details(db, trip_details.id).await else {
+                // This trip_details belongs to a planned_event, no need to do anything
+                continue;
+            };
+            let pot_coxes = TripWithUserAndType::from(db, trip.clone()).await;
+            let pot_coxes = pot_coxes.rower;
+            for user in pot_coxes {
+                let cox = User::find_by_id(db, trip.cox_id as i32).await.unwrap();
+                let Some(user) = User::find_by_name(db, &user.name).await else {
+                    // User is a guest, no need to bother.
+                    continue;
+                };
+                if !user.has_role(db, "cox").await {
+                    // User is no cox, no need to bother
+                    continue;
+                }
+                if user.id == cox.id {
+                    // User already offers a trip, no need to bother
+                    continue;
+                }
+
+                Notification::create(db, &user, &format!("Du hast dich als Ruderer bei der Ausfahrt von {} am {} um {} angemeldet. Bei allen Ausfahrten zu dieser Zeit sind nun alle Plätze ausgebucht. Damit noch mehr (Nicht-Steuerleute) mitfahren können, wäre es super, wenn du eine eigene Ausfahrt zur selben Zeit ausschreiben könntest.", cox.name, self.day, self.planned_starting_time), "Volle Ausfahrt", None).await;
+            }
+        }
+    }
+
     /// Creates a new entry in `trip_details` and returns its id.
     pub async fn create(db: &SqlitePool, tripdetails: TripDetailsToAdd<'_>) -> i64 {
         let query = sqlx::query!(
diff --git a/src/model/usertrip.rs b/src/model/usertrip.rs
index e2a510c..ff12fbf 100644
--- a/src/model/usertrip.rs
+++ b/src/model/usertrip.rs
@@ -1,6 +1,11 @@
 use sqlx::SqlitePool;
 
-use super::{notification::Notification, trip::Trip, tripdetails::TripDetails, user::User};
+use super::{
+    notification::Notification,
+    trip::{Trip, TripWithUserAndType},
+    tripdetails::TripDetails,
+    user::User,
+};
 use crate::model::tripdetails::{Action, CoxAtTrip::Yes};
 
 pub struct UserTrip {}
@@ -79,6 +84,8 @@ impl UserTrip {
                 None,
             )
             .await;
+
+            trip_details.check_free_spaces(db).await;
         }
 
         Ok(name_newly_registered_person)